Method for producing a monolithic piezo actuator with stack elements, monolithic piezo actuator with stack elements, and use of the piezo actuator

ABSTRACT

A piezo actuator has a stack element and at least one further stack element, wherein each of the stack elements has stacked piezoceramic layers and electrode layers arranged between the piezoceramic layers, each of the electrode layers extends to at least one of at least two lateral surface sections of the stack element which have external metallizations on them, and are connected to the electrode layers such that adjacently stacked electrode layers can have different electrical potentials applied to them indirectly via the external metallizations, the stack elements are arranged above one another to form a monolithic total stack, and connected to one another by at least one connecting layer. The following steps are provided: a) provision of the total stack and b) production of a load-relieving crack in the connecting layer. An electrical connector in the form of a wire can be fitted on the relevant external metallization regions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 11/996,603 filed Jan. 24, 2008, which claims priority to U.S. national stage application of International Application No. PCT/EP2006/007405 filed Jul. 26, 2006, which designates the United States of America, and claims priority to German application number 10 2005 034 814.9 filed Jul. 26, 2005, the contents of which are herby incorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to a method for producing a monolithic piezo actuator featuring stack elements stacked one above the other. In addition a monolithic piezo actuator and a use of the piezo actuator are specified.

BACKGROUND

Piezo actuators are known that consist of a plurality of stack elements arranged one above the other. Each of the stack elements features piezoceramic layers arranged above one another made of a piezoceramic material and electrode layers (internal electrodes) arranged between the piezoceramic layers. With monolithic, i.e. one-piece piezo actuators, the entire arrangement consisting of piezoceramic layers and electrode layers is obtained in a common sintering process. The result is a total monolithic stack consisting of stack elements.

When such piezo actuators are electrically activated for the first time up into the high signal range (field strengths of several kV/mm) the piezoceramic material is polarized. This results in an irreversible length change, the so-called remanent extension. Because of the remanent extension and because of an additional extension occurring on electrical activation of the electrode layers in the operation of the piezo actuator, tensile stresses occur in the total stack. These tensile stresses lead, during the polarization or in the operation of the piezo actuator, to cracks (polarization cracks) occurring for example along a boundary surface between a piezoceramic layer and an electrode layer. Cracks which form branches or propagate in the longitudinal direction of the total stack are especially damaging here. Such cracks inevitably lead to the premature failure of the piezo actuator.

SUMMARY

There exists a need for a method of producing piezo actuators in which excessive tensile stress which could lead to the formation of the polarization cracks described above does not occur either during polarization or in operation.

According to an embodiment, in a method for producing a monolithic multilayer piezo actuator, comprising:

-   a stack element and -   at least one further stack element,     -   wherein -   each of the stack elements comprises piezoceramic layers arranged     above one another and electrode layers arranged between the     piezoceramic layers, -   each of the electrode layers of the respective stack element extends     to at least one of at least two side surface sections of the stack     element, -   external metallizations are arranged on the side surface sections of     the respective stack element and are connected to the electrode     layers of the stack element, such that different electrical     potentials can be applied directly to the electrode layers of the     stack elements arranged adjacently above one another via the     external metallizations, -   the stack elements are arranged above one another into a monolithic     total stack and -   the stack elements are connected to each other with the aid of at     least one connecting layer arranged between the stack elements,

the method may comprise the following steps:

a) Preparation of the total stack and

b) Creation of a load-relieving crack of the connecting layer.

According to a further embodiment, the following further method steps may be executed to create the load-relieving crack: c) Polarization of the stack element and d) Polarization of the further stack element. According to a further embodiment, the electrode layers of the further stack element may be short circuited during the polarization of the stack element. According to a further embodiment, the following further method steps may be executed to create the load-relieving crack: e) Polarization of the total stack and f) Depolarization of one of the stack elements. According to a further embodiment, a part polarization may be carried out for polarization of the stack element and/or for polarization of the further stack element. According to a further embodiment, a compressive stress may be applied to the total stack during the polarization of the stack element and/or during the polarization of the further stack element and/or during the polarization of the total stack and/or during of the depolarization of one of the stack elements.

According to another embodiment, a monolithic piezo actuator, may comprise a stack element, and at least one further stack element, wherein each of the stack elements comprise piezoceramic layers arranged above one another and electrode layers arranged between the piezoceramic layers, each of the electrode layers of the respective stack element extend to at least one of at least two side surface sections of the stack element, external metallizations being arranged on the side surface sections of the respective stack element and being connected to the electrode layers of the stack element, such that different electrical potentials can be applied directly to the electrode layers of the stack elements arranged adjacently above one another via the external metallizations, the stack elements being arranged above one another into a monolithic total stack and the stack elements being connected to each other with the aid of at least one connecting layer arranged between the stack elements, wherein the connecting layer comprises a load-relieving crack, the external metallizations of the stack elements form external metallization regions of the total stack which are separated from each other by the connecting layer, and external metallization regions, with the aid of which the same electrical potentials are to be applied to electrode layers of the stack element, are connected electrically-conductively to each other via electrical bridging means.

According to a further embodiment, at least one of the stack elements may have a stack element height selected from the range 1 mm up to and including 10 mm and especially from the range 3 mm up to and including 5 mm. According to a further embodiment, the total stack may have an total stack height selected from the range 10 mm up to and including 200 mm.

According to another embodiment, a method of operating of a such piezo actuator may comprise the step of using the piezo actuator for activation of a valve in particular and especially of an injection valve of an internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained below in greater detail with reference to an exemplary embodiment and the associated FIGURE. The FIGURE is schematic and is not a true-to-scale diagram.

The FIGURE shows a multilayer-construction piezo actuator with a number of stack elements.

DETAILED DESCRIPTION

As stated above, according to an embodiment, a method for producing a piezo actuator in a monolithic multilayer construction is specified, featuring a stack element and at least one further stack element, with each of the stack elements featuring piezoceramic layers arranged above one another and electrode layers arranged between the piezoceramic layers, each of the electrode layers of the respective stack element extending to at least one of at least two side surface sections of the stack element, external metallizations being arranged on the side surface sections of the respective stack element and being connected to the electrode layers of the stack element such that electrode layers of the stack element arranged adjacently above one another can have different electrical potentials applied directly to them via the external metallizations, the stack elements being arranged one above the other into a monolithic total stack and the stack elements being connected to each other with the aid of at least one connecting layer arranged between the stack elements. The method features the following method steps: a) Preparation of the total stack and b) Creation of a load-relieving crack of the connecting layer. To prepare the total stack the outer metallization regions can be created by structured application of electrically-conductive material. It is also conceivable for an enclosed external metallization made of electrically-conductive material to be applied first. The external metallization regions electrically isolated from one another are produced by removing specific parts of the external metallization.

According to another embodiment, a piezo actuator in a monolithic multilayer construction is also specified, featuring a stack element and at least one further stack element, with each of the stack elements featuring piezoceramic layers arranged above one another and electrode layers arranged between the piezoceramic layers, each of the electrode layers of the respective stack element extending to at least one of at least two side surface sections of the stack element, external metallizations being arranged on the side surface sections of the respective stack element and being connected to the electrode layers of the stack element such that different electrical potentials can be applied directly to the electrode layers of the stack element arranged adjacently above one another via the external metallizations, the stack elements being arranged one above the other into a monolithic total stack and the stack elements being connected to each other with the aid of at least one connecting layer arranged between the stack elements. The connection layer features a load-relieving crack, the external metallizations of the stack elements form external metallization regions of the total stack which are separated from each other by the connection layer, and external metallization regions with the aid of which the same electrical potentials are to be applied to electrode layers of the stack element are connected via an electrical bridging means electrically conductively to each other.

The connecting layer is one of the layers of the stack element or elements. For example the connecting layer is one of the piezoceramic layers. The connecting layer can however also be formed by an electrode layer. The load-relieving crack can run in the connecting layer.

Preferably the load-relieving crack may be produced at a boundary surface between the connecting layer and an adjacent layer.

According to various embodiments, a defined tensile/compressive stress profile can be created in the total stack. As a result of the defined tensile/compressive stress profile load-relieving cracks are produced at specific points of the total stack, namely at the boundaries between the stack elements. At these points polarization cracks are induced and promoted. The induced polarization cracks lead during polarization and during operation to a reduction in the maxima of the tensile/compressive stress profile in the total stack. With the load-relieving cracks tensile/compressive stresses arise in the total stack that are lower than the tensile/compressive stresses in a total stack without the load-relieving cracks. As a result the likelihood of the formation of further cracks is reduced. There is no resulting uncontrolled formation and uncontrolled growth of cracks.

The induced load-relieving cracks as a rule propagate in parallel to the electrode layers and the piezoceramic layers in the direction of the surface or surface section of the total stack. This can lead to a break in the external metallization applied to the surface section. The fact that the load-relieving cracks are incorporated in a defined manner means that the points are also known at which there is a high probability of the external metallization being interrupted. The interruption causes external metallization regions to form. The electrical interruption is overcome with the aid of the electrical bridging means. The bridging means functions as an external electrode. The external metallization regions contacted with the bridging means are electrically-conductively connected to each other and can thus be subjected to the same electrical potential. A bridging means suitable for this purpose is for example a bond wire soldered to the external metallization regions. Other bridging means, for example a thin metal plate, a metal track or a wire strand are likewise conceivable.

In accordance with a particular embodiment, the following further method steps are executed to create the load-relieving crack: c) Polarization of the stack element and d) Polarization of the further stack element. Only one of the stack elements is initially polarized. For this purpose only the electrode layers of the stack element to be polarized are electrically activated. The same potential (e.g. ground) is applied to the electrode layers of the other stack element. Preferably the electrode layers of the further stack element are thus short circuited during of the polarization of the stack element. As a result of the change in length which occurs through the polarization of only the stack element, very high tensile stresses result at the connecting layer between the stack element and the further stack element. The load-relieving crack is formed as a result of this tensile stress. Subsequently the further stack element is polarized. During the polarization of the further stack element the electrode layers of the stack element are short circuited.

In accordance with a further embodiment the following further method steps are executed to create the load-relieving crack: e) Polarization of the total stack and f) Depolarization of one of the stack elements. The total stack is first polarized. One of the stack elements is then depolarized. This leads to change in length of the stack element. Once more mechanical tensile stresses are produced which lead to a load-relieving crack of the connecting layer.

The described polarization steps can lead to an almost complete polarization of the piezoceramic material. For example this relates to method step e) with the polarization of the total stack.

However there can be provision for only a part polarization. Thus, in accordance with a further embodiment, a part polarization is executed for polarization of the stack element and/or for polarization of the further stack element.

During the individual polarization steps any parameters can be varied that promote the formation of the load-relieving crack or that make it easier to incorporate the load-relieving crack at a defined location. For example a temperature gradient is created in the total stack during the polarization. Preferably however during of the polarization of the stack element and/or during the polarization of the further stack element and/or during of the polarization of the total stack and/or during the depolarization of one of the stack elements a compressive stress is applied to the total stack. The compressive stress applied influences the change in length created during polarization or depolarization and thereby the formation of the load-relieving crack. The stack is pre-stressed. A pre-stressing pressure can amount to as much as 50 MPa in such cases. The pre-stressing is undertaken uniaxially in the stack direction. Isostatic pressure application is likewise possible.

Through the connecting layer with the load-relieving crack the total stack of the piezo actuator is divided up into at least two stack elements. The fact that the total stack is divided up into smaller stack elements means that smaller mechanical stresses automatically occur with the electrical activation of the piezo actuator.

At least one of the stack elements in this case has a stack element height selected from the range from 1 mm up to and including 10 mm and especially a height selected from the range 3 mm up to an including 5 mm. Within these limits mechanical stresses arising from the electrical activation of the electrode layers can be managed particularly well. At the same time a relatively high stack and thereby a relatively high deflection of the piezo actuator are possible. In this case especially a total stack with stack height is obtainable which is selected from a range 10 mm up to and including 200 mm. The result is a total stack which has a high deflection, a sufficient force transfer and despite its load-relieving crack a very high rigidity.

Thus, a new, reliable piezo actuator is made available. This new piezo actuator can be used for activation of a valve and especially of an injection valve of an internal combustion engine. The internal combustion engine is for example an engine of a motor vehicle.

The piezo actuator 1 is a piezo actuator with a stack (total stack) 10 in a monolithic multilayer design. The piezo actuator 1 consists of a stack element 11 and at least one further stack element 12. The stack elements are arranged one above the other. Between the stack elements is a connecting layer 13 with a load-relieving crack not shown in the FIGURE.

The connecting layer 13 is an electrically non-contacted electrode layer. As an alternative to this the connecting layer 13 is a piezoceramic layer. The total height 103 of the total stack 10 in the stack direction 101 amounts to 30 mm. The stack element heights 113 and 123 of the stack elements 113 and 123 amount to around 2 mm in each case.

Each of the stack elements 11 and 12 consists of piezoceramic layers 111 or 121 respectively made of PZT and electrode layers 112 or 122 respectively made of copper. In alternate embodiments the electrode layers consist of silver or of a silver-palladium alloy respectively. In the total stack 10 or in the stack elements 11 and 12 respectively adjacent electrode layers are routed to different side regions of the total stack 10 or of the stack elements 11 and 12 respectively. A metallization 14 is applied there in each case for electrical contacting of the electrode layers. The FIGURE indicates one of the metallizations 14. The metallization 14 features two metallization regions 141 and 142 each for one of the stack elements 11 and 12. The metallization regions 141 and 142 are connected to each other electrically-conductively via an electrical bridging means 143 in the form of a soldered-on wire.

To manufacture the piezo actuator 1 or the total stack 10 of the piezo actuator 1 ceramic green bodies are printed with electrode material, stacked above one another and laminated under single-axis pressure. Subsequently the “green” stack obtained in this way is released and sintered. This temperature treatment stage (cofiring) leads to the monolithic total stack 10. Subsequently the external metallization 14 is applied to the surface sections 104 of the total stack. In accordance with a first embodiment the metallization is applied to the total surface section 104 and subsequently divided up by removing the metallization into external metallization regions 141 and 142. These external metallization regions are electrically isolated from one another and can be individually electrically activated. As an alternative to this the external metallization is applied in a structured manner. I.e. the external metallization is applied in the form of the external metallization regions isolated electrically from each other. A subsequent removal of the metallization for forming the external metallization regions is not necessary.

A total stack 10 produced in this way or in a similar way is subsequently polarized. To this end only the electrode layers 112 of the stack element 11 are electrically activated. The electrode layers 122 of the further stack element 12 are short circuited. The electrical activation of the electrode layers 122 of the stack element 11 only produces a change in length of this stack element 11. This means that mechanical stresses arise between the stack element 11 and the further stack element 12, which lead to a crack in or on the connecting layer 13. The load-relieving crack forms. Subsequently the total stack is polarized, i.e. the electrode layers 122 of the further stack element 12 are electrically activated.

As an alternative to the demonstrated polarization method the total stack is polarized in a first step. The electrode layers 112 of the stack element 11 and the electrode layers 122 of the further stack element 12 are electrically activated.

Subsequently one of the stack elements 11 or 12 is depolarized. This leads to a different length change of the stack element 11 and of the further stack element 12 and thus to the establishment of the mechanical stresses which lead to the load-relieving crack of the connecting layer 13.

Further embodiments are produced by a compressive stress in the stack direction 101 of the total stack 10 being applied during the polarization of the stack element 11, the polarization of the total stack 10 and/or during depolarization.

This new piezo actuator 1 is used for activation of an injection valve in an engine of a motor vehicle. 

1. A method for producing a monolithic multilayer piezo actuator, comprising: a stack element and at least one further stack element, wherein each of the stack elements comprises piezoceramic layers arranged above one another and electrode layers arranged between the piezoceramic layers, each of the electrode layers of the respective stack element extends to at least one of at least two side surface sections of the stack element, external metallizations are arranged on the side surface sections of the respective stack element and are connected to the electrode layers of the stack element, such that different electrical potentials can be applied directly to the electrode layers of the stack elements arranged adjacently above one another via the external metallizations, the stack elements are arranged above one another into a monolithic total stack and the stack elements are connected to each other with the aid of at least one connecting layer arranged between the stack elements, the method comprises the following steps: a) Preparation of the total stack and b) Creation of a load-relieving crack of the connecting layer.
 2. The method according to claim 1, wherein the following further method steps being executed to create the load-relieving crack: c) Polarization of the stack element and d) Polarization of the further stack element.
 3. The method according to claim 2, wherein the electrode layers of the further stack element being short circuited during the polarization of the stack element.
 4. The method according to claim 1, wherein the following further method steps being executed to create the load-relieving crack: e) Polarization of the total stack and f) Depolarization of one of the stack elements.
 5. The method according to claim 2, wherein a part polarization being carried out for polarization of the stack element and/or for polarization of the further stack element.
 6. The method according to claim 2, wherein a compressive stress being applied to the total stack during the polarization of the stack element and/or during the polarization of the further stack element and/or during the polarization of the total stack and/or during of the depolarization of one of the stack elements. 7-10. (canceled)
 11. A method for producing a monolithic multilayer piezo actuator, comprising: providing a stack element and at least one further stack element, arranging piezoceramic layers of each of the stack elements above one another and electrode layers between the piezoceramic layers, wherein each of the electrode layers of the respective stack element extends to at least one of at least two side surface sections of the stack element, arranging external metallizations on the side surface sections of the respective stack element and connecting the external metallizations to the electrode layers of the stack element, such that different electrical potentials can be applied directly to the electrode layers of the stack elements arranged adjacently above one another via the external metallizations, arranging the stack elements above one another into a monolithic total stack and connecting the stack elements to each other with the aid of at least one connecting layer arranged between the stack elements, and creating a load-relieving crack of the connecting layer.
 12. The method according to claim 11, wherein the following further method steps being executed to create the load-relieving crack: polarizing the stack element, and polarizing the further stack element.
 13. The method according to claim 12, wherein the electrode layers of the further stack element being short circuited during the polarization of the stack element.
 14. The method according to claim 11, wherein the following further method steps being executed to create the load-relieving crack: polarizing the total stack, and depolarizing one of the stack elements.
 15. The method according to claim 12, wherein a part polarization being carried out for polarization of the stack element and/or for polarization of the further stack element.
 16. The method according to claim 12, wherein a compressive stress being applied to the total stack during the polarization of the stack element and/or during the polarization of the further stack element and/or during the polarization of the total stack and/or during of the depolarization of one of the stack elements. 